Polarization sensitivity is a dominant characteristic in present guided wave optical devices. When presented with lightwave signals in different directions of wave polarization such as TE and TM, guided wave optical devices tend to respond differently. In general, this problem arises because each orthogonal polarization experiences different optical properties in the guided wave device such as refractive index, absorption, and changes of these quantities and the like when the device is subjected to an applied field.
This problem is better understood in terms of present optical communication systems in which, for most applications, standard single-mode optical fibers transport lightwave signals from point-to-point. Single-mode optical fibers may be polarization preserving or polarization non-preserving. The latter variety of fiber is widely used because of its lower cost and ease of manufacture. Unfortunately, such fibers do not preserve any particular direction of polarization for the lightwave signals carried by the fiber. As a result, linearly polarized lightwave signals applied at the input end of the fiber emerge at the output end of the fiber in an arbitrary elliptical polarization which varies with time. Under these circumstances, a single polarization guided wave device such as a switch would yield unacceptably high crosstalk and loss due to polarization sensitivity whenever the received lightwave signal exhibits a polarization different from the polarization expected by the switch.
Polarization sensitivity has been effectively handled in directional coupler devices fabricated from birefringent materials such as lithium niobate and lithium tantalate. In such directional coupler devices, two different approaches have been successfully applied. First, a number of guided wave elements have been combined to realize a guided wave device which is polarization independent while the guided wave elements individually exhibit polarization sensitivity. See, for example, a tunable wavelength filter in U.S. Pat. No. 4,390,236. Polarization insensitivity (independence) has also been achieved where the guided wave device is designed having a specialized waveguide geometry for use over an appropriate operating regime. See, for example, a directional coupler switch/modulator in U.S. Pat. No. 4,243,295. Clearly, these approaches influence the birefringence of the waveguide material by external means without attempting or even suggesting the desirability of attempting to change the birefringence intrinsic to the waveguide material.
For semiconductor devices, semiconductor waveguide structures have been realized using one or more quantum wells in the main guiding region. Quantum wells are particularly attractive for semiconductor device structures because they are operable at low voltage which is important in switching and modulation applications and they permit fabrication of relatively compact structures so that comparatively large optical and electrooptic effects are achieved over a short interaction length. See, for example, an electrorefraction directional coupler switch in Appl. Phys. Lett., 55 (22), pp. 2280-2 (1989) and a phase modulator in Appl. Phys. Lett., 52 (12), pp. 945-7 (1988). However, semiconductor quantum well waveguide structures have anisotropic optical properties for different polarizations of lightwave signals propagating in the waveguide at a given wavelength. See the latter reference cited above at FIGS. 1 through 4 therein. This anisotropy arises because the superlattice potential splits the valence band degeneracy thereby creating two separate exciton valence bands, namely, a light hole exciton valence band and a heavy hole exciton valence band. Selection rules for band splitting are described in "Physics and Applications of Quantum Wells and Superlattices," (E. Mendez and K. von Klitzing ed. 1987). Lightwave signals polarized in the plane of the quantum well layers (TE) experience different optical effects than orthogonal lightwave signals which are polarized perpendicular to the plane of the quantum well layers (TM). Published articles have shown that band splitting and, therefore, polarization dependent effects such as absorption occur for lattice matched (unstrained) quantum well layers as well as for lattice mismatched (strained) quantum well layers. See, for example, Phys. Rev. Lett., Vol. 60, Number 5, pp. 448-51 (1988). Presently, there has been no known effort to alleviate the difficulties of polarization dependence in semiconductor quantum well waveguide devices.